1. Technical Field
The present invention relates to a method for producing an element having a fine structure, such as an optical element.
2. Related Art
There are known diffractive optical elements that diffract incident light through a concave and convex structure provided on a surface of the element. The inventors of the present invention consider adding a function of preventing reflection of incident light to the diffractive optical element by providing convex portions much finer than the concave and convex structure in the known diffractive optical elements. Additionally, the inventors of the invention also consider adding a polarization separation function to the diffractive optical element by forming fringe-shaped convex portions (a wire grid) much finer than the concave and convex structure. Thus, when the diffractive optical element includes the concave and convex structure for diffraction and the fine convex portions for the polarization separation function that are superimposed with each other, a range of use of the element can be greatly broadened.
In many cases, the fine convex portions serving to perform the reflection preventing function or the polarization separation function are formed by a photolithographic technique using a photosensitive film. In the technique, the photosensitive film is formed to cover the concave and convex structure on a substrate, exposed to light, and developed so as to form a mask with a fine pattern. Then, etching is performed using the mask, thereby forming the fine convex portions. However, when using a conventionally known technique (e.g. spin coating or spray coating) to form the photosensitive film, a presence of the concave and convex structure under the fine convex portions allows a surface of the photosensitive film to be often made uneven. This seems mainly due to that a liquid material of the photosensitive film remains in the concave portions of the concave and convex structure. It is thus difficult to suitably form a fine structure including the concave and convex structure and the fine convex portions superimposed on the structure. That problem exists not only in production of the diffractive optical element but commonly occurs in production of a fine structure similar to the diffractive optical element. Additionally, the same technological problem can occur in formation of fine convex portions on a substrate simply having an uneven surface as well as in additional formation of fine convex portions on a concave and convex structure provided to perform some function.
JP-T-2002-520677 is an example of related art. The document discloses an example of a fine structure including fine convex portions superimposed on a concave and convex structure. However, regarding a method for producing the fine structure, there is neither sufficient disclosure nor suggestion Thus, the above technological problem has not been solved.